<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
</head>
<body>
    <script>
    function a(){
       a=100;
       console.log(a);              
    }
    a();
    console.log(a);
    //a没有定义，直接使用，则为全局变量
    </script>
</body>
</html>